The present invention relates to electric power meters and, more particularly to an electric power meter that incorporates a current transformer.
Monitoring electrical energy usage is a fundamental function within any electric power distribution system and a primary concern of both consumers and providers of electric power. Electrical energy may be monitored for purposes of usage, equipment performance, and power quality. Volts, amps, watts, vars, power factor, harmonics, kilowatt hours, kilovar hours and other power related measurement parameters are commonly monitored. Typically, the voltage and current, measured at a location within the electric power distribution system, are used to determine the electrical parameters at that location.
Electronic metering of electrical energy typically relies on independent sensing of the load current and the supply voltage. These two quantities are continuously multiplied to calculate the instantaneous load power. Integrating the varying instantaneous load power with respect to time derives the accumulated energy usage. In general, supply voltage sensing can be accomplished with a resistive voltage divider. Load current sensing is more problematic, but is commonly performed with a current transformer.
A current transformer is connected to an electrical system so that the load current (or a shunt current representative of the load current) will flow through the primary winding of the transformer. Often a conductor of the load current is routed through the center of a toroidal current transformer core forming a single turn primary winding of the transformer. The secondary winding typically comprises multiple turns of wire wrapped around the cross-section of the toroidal core. The current in the secondary winding or secondary current is effectively driven from a constant current generator and produces a voltage in an instrument resistor. This voltage can be used to precisely measure the secondary current providing the basis for calculating the corresponding load current flowing in through the primary winding. Ideally, the secondary current is precisely equal to the load current in the primary winding divided by the number of turns in secondary winding. However, actual transformers are not ideal transformers and the magnetization of the core of the current transformer produces errors that reduce the accuracy of the readings produced by the meter.
Current transformer error comprises a phase error and a ratio error. Part of the current in the primary winding is used to magnetize the transformer core with the result that the secondary current is less than the product of the primary current and the ratio of turns in the primary and secondary windings (turn ratio). The ratio error (re) varies with the magnitude of the primary current (I1) as follows:re(%)=K3+K4(log I1)  (1)
where K3 and K4 are constants.
The effect of the ratio error is to alter the relationship between the magnitudes of the measured secondary current (I2) and the primary current (I1) from the theoretical relationship, that is:I1=I2(n)  (2)
where n=turns ratio, to the relationship:
                              I          1                =                              I            2            ′                    ⁡                      (                          n              +                                                nr                  e                                100                                      )                                              (        3        )            
where I′2=measured secondary current
The magnitude of the measured secondary current (I2′) is related to the theoretical secondary current (I2), as follows:
                              I          2                =                              I            2            ′                    ⁡                      (                          1              +                                                r                  e                                100                                      )                                              (        4        )            
In addition, the magnetization of the transformer core and windings causes a phase shift between the current in the primary winding and the current in the secondary winding. The resulting phase error (P) varies with the magnitude of the primary current (I1) approximately according to the relationship:P=K1+K2(I1−M)  (5)
where M, K1 and K2 are constants
In practice M is often approximately equal to ½ and, consequently, a square root approximation can often be conveniently employed as part of the overall correction algorithm.
The values of the constants K1, K2, K3, and K4 depend upon the configuration of the particular current transformer. Factors such as core material and turns ratio affect the values of the constants which are typically ascertained by experiment with samples of a given core configuration. Typically, the values of K1, K2, K3, and K4 are determined for a particular transformer configuration or production batch by comparing the actual performance of a sample of the transformer configuration to the performance of a standard device when the secondary winding is connected in parallel to a particular impedance or burden.
Electronic electric power meters typically incorporate a data processing system, such as a microprocessor or programmable logic controller, to calculate the energy consumption from the measured secondary current values and the supply voltage. It is possible to substantially compensate for the phase error and ratio error by evaluating the error constants for the transformer configuration used in the meter and applying appropriate phase and ratio error correction factors when the instantaneous primary or load current is calculated by the meter's data processing system from a secondary current sample. However, the phase and ratio errors vary with the burden and the magnitude of the primary current. To obtain accurate results, the phase and ratio error correction factors must be available for all possible values of the instantaneous current in the meter's operating range when the meter is operated with an actual burden equal to the test burden with which the meter was calibrated. The error correction factors for a current transformer core configuration are commonly stored in the form of a table, a mathematical formula, or another form representing error correction as a function of the instantaneous primary current. Substantial data storage capacity is necessary to store the required data for correcting currents throughout the meter's operating range and substantial processing power is required to apply the appropriate correction factors to each of the instantaneous load currents calculated from the secondary current samples. In addition, the error correction factors are generated from a sample of a particular transformer configuration and are not necessarily representative and accurate for a specific transformer used in a particular meter.
What is desired, therefore, is a method of accurately determining load current in the primary winding of a current transformer while reducing the computational and data storage resources required for an electric power meter.